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“Would you believe it? The relevance of demeanour in assessing the truthfulness of witness testimony” – Supreme Court

• Nov 2, 2022
• Latest Journal

“Would you believe it? The relevance of demeanour in assessing the truthfulness of witness testimony” - Lord Leggatt

Introduction
I am flattered to be asked to speak to you this morning about assessing the credibility of witnesses at a trial. I appreciate the irony of the invitation. Deciding whether witnesses should be believed is one of the most important tasks that judges have to perform. But it is not one of the competencies required for my job, which consists entirely of deciding points of law on appeal. Certainly, I have seen my fair share of witnesses give evidence when I was a barrister and a first instance judge. But I make no claim to have acquired any special expertise in assessing credibility.

What I do bring to the topic is a somewhat nerdish enthusiasm for popular books about psychology - books such as Daniel Kahneman’s Thinking Fast and Slow and by authors such as Daniel Ariely, Steven Pinker and, in a more journalistic vein, Malcolm Gladwell and Matthew Syed. I expect that many of you have read books of this kind. The best ones do an excellent job of communicating to the general reader notable and often surprising discoveries of psychological science. In the last, let us say, 40 years - the span of my career as a lawyer - there has been a vast amount of research carried out into human cognition and behaviour - research which has yielded many significant findings. Some of these findings seem to me to be of real relevance for legal decision- making - including the evaluation of witness testimony. But the lessons of this research have been slow to impinge on the consciousness of lawyers.

The reduced role of witness testimony Traditionally, calling witnesses to testify orally to what they have themselves observed has been regarded as the primary - and often the only permissible - method of proving facts at a trial. In the heyday of this approach, any other source of information was branded as “hearsay” and was either excluded altogether or was considered an inferior form of evidence when exceptionally it was admitted. Before I come to my main topic, it is worth noting the revolution in how facts are proved at trials brought about by developments in technology. In an age when more and more information is recorded, often electronically, the best evidence in most cases is now hearsay evidence. Documents (using that term in its broadest sense to include any kind of record of information) generally provide more contemporaneous and reliable evidence than the assertions of witnesses testifying in court, often several years after the relevant events.2 I have no doubt that the displacement of witness evidence by documentary evidence is a trend that will continue with further technological advance.

Nevertheless, unless and until absolutely everything that we do and say is recorded, there will remain cases in which courts depend to a greater or lesser extent on witness evidence to establish facts. It is a remarkable feature of the human brain that people can recall and report past experiences as accurately as they often do. But witness evidence has drawbacks that documentary evidence does not. One of them is the fallibility of human perception and memory. Another is that, of course, not all witnesses give honest evidence. Sometimes they lie.

Psychological research has much to teach us about both these limitations of witness testimony. In the case of Gestmin v Credit Suisse in 2013 I drew attention to some of the lessons of psychological research into the nature of memory.3 Today I want to highlight another area of psychological research which I believe has important implications for the assessment of witness evidence. This is research into deception and lying.

Read the full paper here.